Recently, one idea under consideration in 3GPP is that providing a home base station (HeNB) for use indoors in a cell formed by a regular base station (eNB), or a macro cell, to form a CSG cell. The HeNB has a function of regulating user equipment (UE) that may be allowed to access the HeNB, and the UE can only access an HeNB for which the UE has access permission. Accordingly, even when the UE detects an HeNB, the UE cannot connect to the HeNB without access permission no matter how good reception quality the HeNB has.
To obtain access permission, the UE must examine a cell identification (cell ID) included in system information transmitted from the HeNB. Cell IDs include a physical cell ID (PCI) that locally identifies an eNB, a cell global ID (CGI) that uniquely identifies every eNB, and the like. In this case, the latter, COI, is used to uniquely identify the eNB. The UE checks a CGI received in the system information against a list of CGIs of accessible HeNBs, or whitelist, notified from a network. If the CGI owned by the detected HeNB is included in the whitelist, then it is determined that the HeNB is accesible.
The UE may connect to the CSG cell to obtain services specific to the CSG cell such as high-speed, high-quality communication and low price communication. As long as the UE has access permission, therefore, it is desirable that the UE is controlled to connect to the CSG cell (i.e. handed over from the eNB to the HeNB) as much as possible.
When an UE is handed over between eNBs, the UE first measures reception quality of signals from eNBs around the UE, and when the UE detects an eNB of good reception quality, the UE tries to perform a handover to the eNB. At the time when the UE is handed over from an eNB to an HeNB, however, the handover cannot be accomplished only by detecting an HeNB of good reception quality, and it is still necessary to perform a process of checking the presence of access permission for the HeNB of handover destination.
Specific details of a conventional process of a handover to a CSG cell will now be described below with reference to drawings. FIG. 31 is a diagram for illustrating an arrangement of a macro cell and CSG cells, and FIG. 32 is a sequence diagram showing a handover to a CSG cell. Here, description will be made by way of illustration to a case where an accessible CSG cell (PCI=35, CGI=305) is present near a UE camping on a base station (eNB) of a macro cell as shown in FIG. 31.
In this case, as shown in FIG. 32, the eNB first transmits a setting (measurement control) signal necessary for reception quality measurement to the UE (S100). The UE measures reception quality of signals from nearby base stations (eNB and HeNB) based on the setting signal (S101). In this measurement, PCIs of eNBs and HeNBs located around the UE are detected. The UE also has a function of distinguishing whether the detected base station is an eNB or an HeNB based on the PCIs.
Next, the UE transmits a result notification of reception quality measurement (measurement report) to the eNB (S102). In this case, if a certain condition between the reception quality of the signal from the base station (eNB) to which the UE is currently connected and that from a nearby base station (HeNB) is satisfied (for example, the reception quality of the HeNB exceeds that of the eNB), the UE notifies the eNB of the reception quality of the eNB and that of the HeNB that has better reception quality along with the PCI of the HeNB as a measurement result notifying signal (measurement report).
Subsequently, the eNB transmits setting information (measurement configuration) necessary for access verification to the UE (S103). Even when the PCI included in the measurement report is that of the HeNB, other HeNBs, if any as shown in FIG. 31, that have the same PCI prevent the eNB from distinguishing which HeNB is the nearby base station (HeNB) notified in the measurement report. Therefore, the UE receives a CGI, which is a unique cell identification, from system information transmitted from the HeNB, checks the CGI against the whitelist retained by the UE for access verification, and then measures the reception quality of the HeNB again (S104).
The UE then transmits a result notification of the access verification (measurement report) to the eNB (S105). In this case, the measurement report notifies the result of the reception quality remeasurement of the HeNB along with the CGI of the HeNB.
As another way of checking the presence of access permission of the detected HeNB, it is conceivable to refer to history information (fingerprints) of accessible HeNBs that have been previously detected, if any retained by the UE, to determine whether or not the detected HeNB is accessible. The history information includes, for example, a cell identification such as a PCI and a CGI, positional information of the HeNB (GPS information), and the like. In this case, in the step S102, a measurement report is issued to the eNB only for an HeNB that satisfies a certain condition and that the UE has access permission. Therefore, the steps S103 to S105 may be omitted.
When the UE has access permission for the detected HeNB, the eNB transmits a handover request to the HeNB (S106). In this case, the eNB transmits the handover request to a base station of handover destination (the HeNB) via an HeNBGW, which is an access gateway, or MME. When the HeNB that receives the handover request determines that the handover of the UE may be allowed, the HeNB transmits a handover response signal to the eNB via HeNBGW or MME (S107).
Upon receiving the handover response, the eNB transmits a handover command signal to the UE (S108). The handover command includes an identification of the UE for the HeNB (C-RNTI) and information necessary for the UE to perform uplink synchronization. Upon receiving the handover command, the UE transmits a random access preamble to the HeNB to perform the uplink synchronization (S109). Upon receiving the random access preamble, the HeNB performs uplink allocation for the UE and notifies the allocation information to the UE. Once the UE is successfully connected to the HeNB, then the UE transmits a handover confirmation signal to the HeNB (S111) notifying that the handover has been completed in the UE. Such handover control to a CSG cell is proposed by 3GPP or the like (for example, see Non Patent Literature 1).
In 3GPP, incidentally, a mixed carrier environment where a CSG cell and a macro cell use the same frequency is under consideration. When the same frequency is used, there may be a problem of interference between a signal from a UE connected to the CSG cell and that from another UE connected to a macro cell. The amount of uplink (UL) interference varies depending on the location of the UE in the macro cell. For example, when the UE is located near an eNB, the macro cell is much affected by the UL interference, and when the UE is located at a macro cell edge, the macro cell is less affected by the UL interference.
As described above, it is desirable that the UE is controlled to connect to the CSG cell as much as possible, as long as the CSG cell is accessible. Conventionally, therefore, an offset is added to the measurement result of reception quality of an accessible HeNB when the reception quality is measured. With this offsetting, an HeNB is more likely to be selected as a handover candidate; in other words, the UE is controlled to perform a handover preferentially to the HeNB.
Conventional offsetting will now be described in detail with reference to the drawing. FIG. 33 shows conventional offsetting. For example, in common handover control, a handover is initiated if the following Formula 1 is satisfied as a result of reception quality measurement:Reception quality of eNB<reception quality of HeNB.  (Formula 1)
In contrast, in handover control with offsetting, an offset value (positive offset value) is added to reception quality of an HeNB as in the following Formula 2 so that the UE is more likely to be handed over to the HeNB, as shown in FIG. 33 (for example, see Non Patent Literature 2):Reception quality of eNB<reception quality of HeNB+offset value.  (Formula 2)